Abstract: This article explains the difference between grounded
and ungrounded electrical systems. In a grounded system, a ground fault
will cause the circuits to open. In an ungrounded system, a single ground
fault will not interrupt the circuit. Ungrounded systems pose a hazard though,
because another ground fault in a different phase of the system occurring
at the same time could cause a phase- to-phase short. The National Electric
Code suggests using ground fault detectors with ungrounded systems.

This article also lists the National Electric Code requirements for grounding
wires. Every interior wiring system with alternating current rated 50 volts
to 1,000 volts should be grounded. The article contains the individual specifications
for different volt levels.